A two-stage process of cancer induction has been reported in several diverse biological systems, and suggested to be a general feature of the mechanism of carcinogenesis by chemicals. Most experiments have used different agents to induce the initiation and promotion phases and the question of the identity of the promoting stimulus in cancer by a single chemical agent has received little attention. The similarities in response to tumor-promoting agents of mouse skin tumorigenesis and Chinese hamster V79 cell mutagenesis systems suggest that the latter may be suitable for addressing the above question. V79 cells do not have metabolic enzymes, required for conversion of precarcinogens to initiators, but this activity can be supplied exogenously as required; therefore clear separation of initiating and promoting activity of precarcinogens or their metabolites is possible using this cell type. The clear separation of other effects on a cell type due to the parent hydrocarbon from those of its metabolites is similarly possible. Up to the present the use of the V79 system has demonstrated the separaton of polycyclic hydrocarbons into two groups on the basis of cytotoxicity, and further that the compounds 3-methylcholanthrene (3-MC) and benzo(a)pyrene (B(a)P) have different effects on survival and mutagenesis in previously mutagenized cells. 3-MC was without effect on either parameter in up to high concentrations, suggesting it is without promoting activity, a proposal recently made by others from observations in another system (Scribner & Scribner, Abstracts of Symposium on Cocarcinogenesis and Biological Effects of Tumor Promoters, 1980). The continuing study will consist of two phases: a) Further study of the effect of aromatic precarcinogens and weakly- or nonmutagenic carcinogen metabolites on mutagenic metabolite-induced mutation frequencies in V79 cells. b) Investigation of the mechanisms underlying the differential activities of 3-MC and B(a)P towards survival and mutation frequency of promutagenized cells.